1. Field of the Invention
The present invention relates to a thermal printer for printing information on a recording medium such as a label sheet with a thermal head by leading the medium between a platen and the thermal head.
2. Description of the Related Art
A label printer for printing product names or bar codes on labels, for example, comprises a printing section having a platen roller and a line thermal head, and a paper-feeding mechanism for feeding labels to the printing section.
Upon the printing using the line thermal head, a direct-thermal printing or a heat-transfer thermal printing system has been used so far. With the direct-thermal printing, selected portions of a label composed of heat-sensitive paper are directly heated by the line thermal head, thereby to form characters on the label. With the heat-transfer thermal printing, however, selected portions of a transfer ribbon, which put on a label, are heated by the thermal head, thereby to transfer ink of the ribbon onto the label.
Therefore, for performing the transfer-type printing, the printer requires a ribbon supply device having the shafts for supplying and taking up the transfer ribbon, in addition to the printing section and the paper-feeding mechanism.
However, in conventional label printers, the ribbon supply device for the transfer-type printing is integrated with the printer body as a part of the printer. Thus, it is difficult to remove only the ribbon supply device from the printer body in view of the configuration thereof.
Therefore, an expensive ribbon supply device is always attached to the printer body even when the direct-thermal printing system, requiring no transfer ribbon, is used, so that the printer becomes expensive and cannot adequately show the advantage of the direct-thermal printing, which does not require the use of a transfer ribbon.
Generally, the ribbon supply device has a ribbon guide shaft arranged on the rear side of the printing section, a transfer-ribbon take-up shaft, and a transfer-ribbon supply shaft. When the take-up shaft is rotated by a motor, it directly takes up the transfer ribbon passed through the printing section and draws the transfer ribbon from the supply shaft. The supply shaft is connected to a load-adjusting mechanism which brakes the supply shaft so as to apply back tension to the transfer ribbon.
Though the motor torque directly applied to the take-up shaft is constant, the winding force acting on the transfer ribbon depends on the diameter of the roll of the transfer ribbon wound on the take-up shaft. Specifically, the winding force increases as the diameter of the ribbon roll decreases and vice versa. By the variation in the winding force, the transfer ribbon is not taken up with a constant force. Therefore, the ribbon is firmly wound on the shaft at the beginning of taking up but it is loosely wound at the end of the taking-up. However, to reuse the once-used transfer ribbon which has unevenly wound up, the ribbon tends to run unstably. Therefore, there is the problem that the once-used transfer ribbon unevenly wound up cannot be reused.
Moreover, at the beginning of the taking-up of the ribbon where a large winding force is applied, disconnection of the ribbon or smudge (phenomenon in which a paper sheet is smudged because the ink layer is peeled from the base film of the ribbon due to excessive tension of the transfer ribbon and attached on the paper sheet) easily occurs. On the contrary, at the end of the taking-up of the ribbon where a small winding force is applied, sticking (phenomenon in which the transfer ribbon moves at speed lower than a set speed, causing faulty transfer) easily occurs. Therefore, there is also the problem that printing quality is impaired due to these troubles.